This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. Section 119 from an application for METHOD FOR DETECTING A HAND-OFF TARGET FREQUENCY IN A CELLULAR MOBILE TELECOMMUNICATION SYSTEM filed earlier in the Korean Industrial Property Office on Mar. 12, 1999 and there duly assigned Ser. No. 8194/1999.
1. Field of the Invention
The present invention relates to a cellular mobile telecommunication system, and more particularly to a method for detecting a hand-off target frequency depending on the availability of radio resources at the base transceiver stations (BTSs) and the location of a mobile station (MS).
2. Description of the Related Art
In a code division multiple access (CDMA) communication system, the whole service area is divided into a plurality of base station coverage areas. Each divided area is defined as a cell and served by a base transceiver stations (BTS). All BTSs are systematically controlled by a mobile switching center (MSC), enabling a roaming MS to maintain the communication link between cells. For greater capacity, each cell is generally divided into several sectors. For example, each cell may be divided into three sectors, xcex1, xcex2, and xcex3, and each sector is provided with a sector antenna for serving the MS.
Referring to FIG. 1, the conventional CDMA communication system comprises a plurality of BTSs 120, 130 and 140 for communicating with the MS 110, a base station controller (BSC) 150 for controlling the BTSs, and a mobile switching center (MSC) 160 for connecting the BSC to another BTS or a public switched telephone network (PSTN). The CDMA system provides a hand-off scheme for maintaining the communication link between an MS and the BTSs when the MS travels from one cell to another.
The CDMA system employs a pseudo-random noise code (PN code) to divide a frequency into multiple code channels, thus serving considerably more subscribers compared to other communication systems such as the frequency division multiple access (FDMA). Basically, all the BTSs of the CDMA communication system employ the same set of frequency channels. However, the BTS located in a dense urban area requires more frequency channels to meet the high demand to serve more subscribers than in other areas. In this case, when an MS communicating with a BTS on a given frequency channel travels to another BTS not having the same frequency channel, the communication channel has be transferred from the current frequency channel of a cell to a different frequency channel of another cell. This is known as a hard hand-off or inter-frequency hand-off. Namely, the hard hand-off replaces the current communication channel with another channel in a different cell so that the MS can maintain the current communication link without interruption.
In the CDMA system, each BTS (or sector) is assigned with a specific pilot signal by which the MS distinguishes each BTS. The MS measures the strength of all the received pilot signals, which are transferred to a BSC through the currently connected BTS. The BSC analyzes the strength of the pilot signals to determine the hand-off and the type of hand-off. This process is called the mobile assisted hand-off (MAHO) because the hand-off is performed based on the signal strength measured by the MS. The IS-95B standard recommendation specifies that the MS would inform the BSC of the time to perform the inter-frequency hand-off as well as the environment of the hand-off target frequency (i.e., the strength of the pilot signal). Accordingly, the MS periodically or upon detecting the target frequency specified by the BSC, based on which the BSC determines whether the target BTS should perform the inter-frequency hand-off. However, when the target BTS has no reserved radio resource that can be assigned to the target frequency, the MS cannot help but lose the current communication connection. For this reason, the MS must often search for another target frequency to assist the hand-off which increases the load on the MS, causing communication link failure or power control failure.
It is an object of the present invention to provide a method for detecting a target frequency to provide an inter-frequency hand-off in a CDMA communication system, thus increasing the success rate of hand-off while minimizing the number of attempts searching for the alternate hand-off target frequency.
According to one embodiment of the present invention, a method for detecting a hand-off target frequency of an MS in a CDMA communication system, comprising the steps of transmitting a pilot strength measurement message (PSMM) from an MS to a BSC through at least one BTS; causing the BSC to determine based on the received PSMM whether the MS is located in the border region of a BTS or whether the forward radio communication channel of the MS is in bad state; causing the BSC to command the MS to detect a specific hand-off target frequency when the MS is located in the border region of the BTS or when the forward radio communication channel is in bad state; causing the MS to report all the BTSs providing the specified hand-off target frequency to the BSC; causing the BSC to perform the inter-frequency hand-off based on the reported target BTSs from the MS; causing the BSC to check whether the MS, while detecting the hand-off target frequency, meets the requirement of stopping the detection for the hand-off target frequency; causing the BSC to command the MS to stop the detection for the hand-off target frequency when meeting the requirement; and causing the MS to stop the detection.
According to another embodiment of the present invention, a method for detecting an inter-frequency hand-off target frequency by an MS in a CDMA communication system, comprising the steps of classifying the frequency channels of the BTSs as a xe2x80x9csufficient frequency resource statexe2x80x9d if these frequency channels are available for a new call assignment or a hand-off; classifying the frequency channels of the BTSs as an xe2x80x9cinsufficient frequency resource statexe2x80x9d if these frequency channels are substantially available and has a probability of failing the assignment of a channel during the periodic detection of a inter-frequency hand-off target frequency; classifying the frequency channels of the BTSs as a xe2x80x9cconsumed frequency resource statexe2x80x9d if these frequency channels are unavailable; detecting common frequency commonly provided by the BTSs listed in the active set; selecting one of the detected common frequencies as the target frequency that renders all the BTSs to be in the xe2x80x9csufficient frequency resource statexe2x80x9d; if there is no common frequency that renders all the BTSs to be in the xe2x80x9csufficient frequency resource statexe2x80x9d, excluding any common frequency corresponding to the xe2x80x9cconsumed frequency resource statexe2x80x9d; selecting a common frequency from the remaining detected frequencies as the target frequency that renders relatively more BTSs to be in the xe2x80x9csufficient frequency resource statexe2x80x9d; and, if no common frequency that renders relatively more BTSs to be in the xe2x80x9csufficient frequency resource statexe2x80x9d, selecting a common frequency from the remaining detected frequencies as the target frequency which renders relatively more BTSs to be in the xe2x80x9cinsufficient frequency resource statexe2x80x9d as the target frequency.
According to still another embodiment of the present invention, the method for performing an inter-frequency hand-off of an MS by a BSC in a CDMA communication system, comprising the steps of classifying the frequency channels of the BTSs as a xe2x80x9csufficient frequency resource statexe2x80x9d if these frequency channels are available for a new call assignment or a hand-off; classifying the frequency channels of the BTSs as an xe2x80x9cinsufficient frequency resource statexe2x80x9d if these frequency channels are substantially available and has a probability of failing the assignment of a channel during the periodic detection of a inter-frequency hand-off target frequency; classifying the frequency channels of the BTSs as a xe2x80x9cconsumed frequency resource statexe2x80x9d if these frequency channels are unavailable; detecting common frequency commonly provided by the BTSs listed in the active set; causing the MS to detect and report all target BTSs providing the target frequency, as specified by the BSC, as well as the strength of pilot signals from the target BTSs; causing the BSC to determine whether to perform the inter-frequency hand-off of the MS according to the report received from the MS; checking whether the target frequency of the target BTSs is in the xe2x80x9cconsumed frequency resource statexe2x80x9d if the MS requires the inter-frequency hand-off; performing the inter-frequency hand-off to the target frequency if no target BTS has the target frequency in the xe2x80x9cconsumed frequency resource statexe2x80x9d; checking whether the target BTS has the maximum pilot strength when it has the target frequency in the xe2x80x9cconsumed frequency resource statexe2x80x9d; performing the inter-frequency hand-off of the MS to the remaining target BTSs while excluding the target BTS in the xe2x80x9cconsumed frequency resource statexe2x80x9d if the target BTS does not have the maximum pilot strength; and, commanding the MS to detect a new target frequency selected when the BTS with the target frequency in the xe2x80x9cconsumed frequency resource statexe2x80x9d has the maximum pilot strength.
According to a further embodiment of the present invention, a method for detecting an inter-frequency hand-off target frequency by an MS in a CDMA communication system, comprising the steps of classifying the frequency channels of the BTSs as a xe2x80x9csufficient frequency resource statexe2x80x9d if these frequency channels are available for a new call assignment or a hand-off; classifying the frequency channels of the BTSs as an xe2x80x9cinsufficient frequency resource statexe2x80x9d if these frequency channels are substantially available and has a probability of failing the assignment of a channel during the periodic detection of a inter-frequency hand-off target frequency; classifying the frequency channels of the BTSs as a xe2x80x9cconsumed frequency resource statexe2x80x9d if these frequency channels are unavailable; detecting common frequency commonly provided by the BTSs listed in the active set; causing a BTS to report to the BSC that it has no frequency resource for a new assignment in the particular frequency channel assigned to itself; causing the BSC to set the BTS as a xe2x80x9cconsumed frequency resource statexe2x80x9d; and causing the BSC to command the MS to detect another target frequency.
The present invention will now be described more specifically with reference to the drawings attached only by way of example.